Bilge pumps, sump pumps and similar DC or AC electrical powered pumps used to pump out accumulated water traditionally use a float switch for the pump power circuit in which the water level raises and lowers the float sufficiently to close and open the associated switch. Such float switch devices require a number of moving parts which wear or bind and eventually fail; and the wearing and binding is often accelerated by the damp, corrosive and dirty environment in which these float switches are used. Failure of the switch can have catastrophic effects since when the pump does not operate the water accumulates and can flood the area. In the case of bilge pumps, the flooding can sink the vessel.
One attempt to eliminate the need for such float switches includes means to periodically, automatically, e.g., every five minutes, turn on the pump whether or not there is water or liquid buildup. The pump current is then monitored, and, if it is low, a no-load condition is detected and the pump is shut off. If the current is normal, a load condition is detected and the pump is permitted to keep pumping until the water is drained and the low current condition reoccurs. See U.S. Pat. No. 5,076,763, "Pump Control Responsive to Timer, Delay Circuit and Motor Current", assigned to the same assignee.
While this solves the float switch problems, it adds another. Namely, in some installations the noise of the pump turning on every five minutes or for a similar time interval annoys owners, passengers and crew. For, even if no water is present the pump still relentlessly makes noise every five minutes. One attempt to overcome this problem as set forth in copending PCT application, International Application No. PCT/US93/09415, "Soft-Start Pump Control System", filed Oct. 1, 1993, by Anastos et al., assigned to the same assignee and incorporated herein by reference, uses a pump control system for periodically yet quietly operating a pump. The system automatically energizes the pump motor at regular fixed test intervals but at reduced power by reducing the power of the test cycle while sensing the motor current and then stepping up the power if the motor current amplitude indicates that there is liquid to be pumped. That approach can be effectively applied to AC motor applications. In the same manner as DC motors, AC motors also exhibit higher levels of current draw when pumping liquids.
However, a problem still exists in that the pump may be coming on at fixed intervals too infrequently to properly drain the liquid when a serious flooding problem is occurring or is coming on much too often when there is little or no liquid present.